Dear Enxio
I do hope that you'll give the Killers another chance, because many of us find them the most satisfying of all these puzzles, particularly because they allow infinite scope for combinations and divisions (and the arithmetic isn't hard (multiples of 45); it's more a question of seeing how the cages fit together, to reveal eliminations). They do require quite different strategies from 'plain' Sudoku but are all the better for that, once you get the hang of them (lots of scribbling in the margins to check the possibilities).
[So maybe that's why they appeal more to pen/pencil/paper/eraser people, rather than those who use spreadsheets/computer solvers?]
Try "killersudoku.online.com" for all levels of difficulty, or
http://entertainment.timesonline.co.uk/ ... es/sudoku/for a range of non-symmetrical examples, increasing in difficulty over each week.
Ruud's Assassins, and those here, are really at the top of the scale and could, I imagine, be quite a frustrating place to start. (They're pretty frustrating altogether!!! but also allow us all to aspire to an ultimate challenge.) They also seem to be designed by real people, not just generated by computer programmes, and so include some element of human engagement with the setter.
All best
s